Polyphenylene ether that is represented by poly (2,6-dimethyl-1,4-phenylene ether) is excellent in heat resistance, self-extinguishing property, chemical resistance, dimensional stability, electric properties, workability, etc. and it is one of the five well-known engineering plastics.
Conventionally, polyphenylene ether, specifically poly (2,6-dimethyl-1, 4-phenylene ether) has been prepared by oxidation polymerization of 2,6-dimethyl phenol in an organic solvent such as toluene and benzene, using tertiary amine (e.g. pyridine etc.) and metal salt (e.g. cuprous chloride etc.) that can form a complex jointly with tertiary amine, as a catalyst at room temperature in oxygen atmosphere. In this oxidation polymerization, however, oxygen is used as an oxidizer, and toluene or the like as a solvent, and thus there is an extremely high risk of explosion hazard so that a large-scale explosion-proof reaction tank is needed. Further, since an organic solution such as toluene or the like is used, organic solvent recovery process also is needed.
In order to solve these problems, inventors of the present invention once disclosed an example of the process for preparing polyphenylene ether, by oxidation polymerization, using water as a solvent, in which potassium hexacyanoferrate (III) or a water-insoluble copper complex is used as an oxidizer (see Patent Document 1). According to this prior art, water with less environmental burdens is used as a solvent to prepare polyphenylene ether, thus enabling the conventional reactor tank to be replaced by one having a substantially simplified explosion-proof and solvent-resistant structure. Further, solvent recovery process can be cut out because target polyphenylene ether is able to be separated by the simple filtering of polymer precipitated in a water phase.
In the meantime, this reaction is heterogeneous polymerization in which 2,6 dimethyl phenol as a monomer remaining dissolved in water is allowed to precipitate as it is converted into a dimer or higher one in the reaction process, and when oxidizer is added to the solvent, polyphenylene ether is immediately allowed to precipitate as a white solid from a water phase. The inventors of the present invention concluded that the foregoing reaction has a peculiar polymerization mechanism in which a radical produced on a water interface grows in a coupling manner in the solid phase (see Non-Patent Document 1)    Patent Document 1: Japanese Unexamined Patent Publication. No. 2004-26959    Patent Document 2: Japanese Unexamined Patent Publication No. 2000-281798    Patent Document 3: Japanese Unexamined Patent Publication No. H5-78471    Patent Document 4: Japanese Unexamined Patent Publication No. H9-118746    Non-Patent Document 1: Angew. Chem. Int. Ed. by K. Saito, T. Tago, T. Matsuyama, H. Nishide, vol. 43, p 730, 2004